With widespread use of the internet, increase in the capacity of an optical communication system has been advanced. For example, for a trunk line system, researches are conducted on an optical transmitter and an optical receiver which can transmit signals beyond 40 Gbit/s per wavelength. With increasing the bit rate per wavelength, also increased is degradation in signal quality due to degradation in tolerance of optical signal to noise ratio (OSNR) and due to waveform distortion caused by wavelength dispersion, polarization mode dispersion and a nonlinear effect of a transmission line. Accordingly, a digital coherent reception method which has a high OSNR tolerance and a high waveform distortion tolerance has attracted attention in recent years.
In the digital coherent reception method, light intensity information and phase information are extracted from a received light signal, and demodulation is performed by a digital signal processing circuit. Accordingly, in the digital coherent reception method, improvement of OSNR tolerance by coherent reception and waveform distortion compensation by the digital signal processing circuit are realized. As a result, by the use of the digital coherent reception method, high reliability can be achieved even in an optical communication system beyond 40 G bit/s.
FIG. 8 shows an example of a configuration of a reception device 9 generally used in the digital coherent reception method and an example of a configuration of an optical receiver 90 provided in the reception device 9. The optical receiver 90 causes a received light signal to interfere with a local oscillator light having almost the same wavelength as that of the received light signal, and converts the interference signal to an electrical signal (coherent detection). Because coherent detection has strong polarization dependence, one optical receiver 90 can receive only an optical signal with the same polarization state as that of the local oscillator light. For this reason, the reception device 9 has a polarization separation circuit in an inputting part for a received light signal, and thereby separates the received light signal into two orthogonal polarization components. Accordingly, to detect one light signal, the reception device 9 requires two optical receivers 90. However, the disadvantage can be compensated by polarization multiplexing the light signal at the sending side and thereby doubling the information transmission amount.
In FIG. 8, in each of the optical receivers 90 provided for detecting the respective polarizations, a received light signal and local oscillator light are inputted to an optical 90 degree hybrid 91. The optical 90 degree hybrid 91 outputs a total of four output light beams which includes a pair of output light beams (I) generated by causing the received light signal interfere with the local oscillator light in in-phase and counter-phase conditions and another pair of output light beams (Q) generated by the interference in quadrature (90 degrees) and counter-quadrature (−90 degrees) conditions. Each of the two pairs of optical signals outputted from the respective optical 90 degree hybrids 91 is converted into a pair of electric current signals by two photodetectors (PDs) 92. Then, on the basis of these current signals, difference output units 93 output signals to be processed by a digital signal processor (DSP) 95 arranged at a following stage. Here, the difference output units 93 are configured by, for example, a differential type transimpedance amplifier. In this case, each of the difference output units 93 efficiently extracts only a beat component between the received light signal and the local oscillator light by canceling out unnecessary direct-current components in the current signals from two PDs 92. An electrical signal outputted from each of the difference output units 93 represent an in-phase interference component (I) or a quadrature interference component (Q) between the received light signal and the local oscillator light.
A total of four electrical signals thus outputted from the optical receivers 90 including I-Q components for X-polarization (XI, XQ) and for Y-polarization (YI, YQ) are AD (analog to digital)-converted into digital information signals at a high speed by analog to digital converters (ADCs) 94. Then, on the digital information signals thus obtained, the DSP 95 performs various equalization/determination processes by the use of digital signal processing widely used in wireless communication. Then, after performing error correction on the signals having undergone the equalization/determination processes, the DSP 95 outputs an information signal of 100 G bit/s. Such a reception device and an optical receiver of the digital coherent receiving method are also described in Patent Document 1.